The present invention pertains to a ferroelectric thin film device that finds applications in pyroelectric infrared sensor devices, piezoelectric devices, electrooptical devices, and capacitors made of ferroelectric materials and to a fabricating process thereof.
Ferroelectrics are substances in which there exists spontaneous polarization by permanent dipoles arranged in a parallel or antiparallel fashion even when there are no electric fields. The direction of such spontaneous polarization is reversed by the application of an electric field from the outside. Materials having ferroelectricity find applications in various types of electronic components such as pyroelectric infrared sensor devices, piezoelectric devices, optical modulators based on electrooptical effects of the ferroelectric material, and non-volatile memory devices. Typical ferroelectric materials are compounds of oxygen having a perovskite-type crystal structure (e.g., PbTiO.sub.3, Pb.sub.1-x La.sub.x Ti.sub.1-x/4 O.sub.3 (PLT), PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT), and BaTiO.sub.3.
There is an application of the ferroelectric in which changes in the spontaneous polarization P.sub.s are extracted as output. For example, in the case of pyroelectric infrared sensor device or in the case of piezoelectric devices, their output is maximized when the spontaneous polarization P.sub.s occurs in one direction only. There are many types of ferroelectrics and they have different properties (e.g., different permittivities and velocities of sound) depending on the crystal axis. Therefore, there have been demands for a technique capable of arranging crystal axes in the same direction, in order to improve the characteristics of the ferroelectric and realize new devices with sophisticated functions. Most of the ferroelectrics, currently used for the infrared sensors and piezoelectric devices, are polycrystalline ceramics which do not have any directionality in their crystal axis arrangement, as a result of which the spontaneous polarization P.sub.s occurs along several crystal axes.
As electronic components have been become smaller and smaller, ferroelectric devices likewise are reduced in size. The advancement of ferroelectric thin film technology has been expected. Particularly, ferroelectric thin film devices incorporating therein epitaxial thin layers or oriented thin layers are now under intensive research.
The crystal axis of ferroelectrics depends very much on the type of substrate. The following has been reported in several papers, for example, in J. Appln. Phys., Vol. 60, p.361 (1986). That is, the crystal axes of PbTiO.sub.3 (or PZT) are oriented in the &lt;001&gt;-direction when a MgO single crystal substrate cleaved on the (100)-plane is used, while it orients in the &lt;111&gt;-direction when a sapphire single crystal substrate is used (i.e., the sapphire's c plane). Conventional ferroelectric thin film devices rely on expensive MgO or sapphire substrates. This increases the fabrication cost of ferroelectric thin film devices thereby increasing the production cost of electronic components employing ferroelectric thin film devices.
Direct epitaxial growth of ferroelectric thin films on the silicon substrate is infeasible, which is an obstacle to integration of high-performance signal processing devices into ferroelectric thin film devices.
Therefore, it is a main object of the present invention to provide an improved ferroelectric thin film device. According to the present ferroelectric thin film device not relying on expensive substrates, the crystal axis can be arranged. As a result, the present invention can not only cut down the production cost but also improve the device performance.